Dynamics of GLP-1R peptide agonist engagement are correlated with kinetics of G protein activation

Where are we now? Biased signalling of Class B G protein-coupled receptor-targeted therapeutics

Class B G protein-coupled receptors (GPCRs) are a subfamily of 15 peptide hormone receptors with diverse roles in physiological functions and disease pathogenesis. Over the past decade, several novel therapeutics targeting these receptors have been approved for conditions like migraine, diabetes, and obesity, many of which are ground-breaking and first-in-class. Most of these therapeutics are agonist analogues with modified endogenous peptide sequences to enhance receptor activation or stability. Several small molecule and monoclonal antibody antagonists have also been approved or are in late-stage development. Differences in the sequence and structure of these therapeutic ligands lead to distinct signalling profiles, including biased behaviour or inhibition of specific pathways. Understanding this biased pharmacology offers unique development opportunities for improving therapeutic efficacy and reducing adverse effects. This review summarises current knowledge on the ligand bias of approved class B GPCR drugs, highlights strategies to refine and exploit their pharmacological profiles, and discusses key considerations related to receptor structure, localisation, and regulation for developing new therapies.

GLP-1 receptor agonist properties of a chimeric peptide derived by hybridization of Latrodectus αLatrotoxin and Heloderma Exendin-4

Chimeric peptides comprised of amino acid sequence motifs found within hormones, neuropeptides, and insect or lizard toxins are now under investigation for their potential use in therapeutics. Here, we report the discovery of one such peptide designated as Black Widow Spider-Exendin-4 (BW-Ex-4). It consists of a putative G protein-coupled receptor (GPCR) binding domain present within αLatrotoxin (αLTX) isolated from Latrodectus, and fused to N- and C- terminal motifs found within the glucagon-like peptide-1 receptor (GLP-1R) agonist Exendin-4 isolated from Heloderma. FRET reporter assays that monitor cAMP production establish BW-Ex-4 to be a specific GLP-1R agonist without any stimulatory action at glucose-dependent insulinotropic peptide (GIP), glucagon, or corticotropin releasing hormone (CRH) receptors. Structural modeling studies of the predicted BW-Ex-4 binding sites at GPCRs of Family B provide new insights concerning the molecular basis for chimeric peptide stimulatory actions at the GLP-1R. We also report that BW-Ex-4 acts in obese hyperglycemic Leprdb/db mice to suppress appetite, lower body weight, improve glucoregulation, and to reduce circulating levels of pro-inflammatory cytokines. Collectively, these findings establish a combinatorial chimeric peptide chemistry in which αLTX serves as a molecular scaffold for the design of hybrid peptides with novel GPCR stimulating properties.

Structural pharmacology and mechanisms of GLP-1R signaling

Glucagon-like peptide-1 receptor (GLP-1R), a class B1 G protein-coupled receptor, plays critical roles in glucose homeostasis. Recent structural pharmacology studies using cryogenic electron microscopy, X-ray crystallography, mass spectrometry, and functional analyses, have provided valuable insights into its activation by endogenous hormones and mono- or dual agonists like semaglutide and tirzepatide, highly effective in treating type 2 diabetes and obesity. They highlight significant conformational changes in the extracellular and transmembrane domains of GLP-1R that drive receptor activation and downstream signal transduction. Additionally, allosteric modulators, supported by emerging structural information, show great promises as an alternative strategy. Future research investigating unexplored effector interactions, biased signaling, weight rebound mechanisms, and personalized therapy strategies will be critical for developing better therapeutic agents targeting GLP-1R.

Biased agonism in peptide-GPCRs: A structural perspective

G protein-coupled receptors (GPCRs) are dynamic membrane receptors that transduce extracellular signals to the cell interior by forming a ligand-receptor-effector (ternary) complex that functions via allosterism. Peptides constitute an important class of ligands that interact with their cognate GPCRs (peptide-GPCRs) to form the ternary complex. "Biased agonism", a therapeutically relevant phenomenon exhibited by GPCRs owing to their allosteric nature, has also been observed in peptide-GPCRs, leading to the development of selective therapeutics with fewer side effects. In this review, we have focused on the structural basis of signalling bias at peptide-GPCRs of classes A and B, and reviewed the therapeutic relevance of bias at peptide-GPCRs, with the hope of contributing to the discovery of novel biased peptide drugs.

Hidden GPCR structural transitions addressed by multiple walker supervised molecular dynamics (mwSuMD)

The structural basis for the pharmacology of human G protein-coupled receptors (GPCRs), the most abundant membrane proteins and the target of about 35% of approved drugs, is still a matter of intense study. What makes GPCRs challenging to study is the inherent flexibility and the metastable nature of interaction with extra- and intracellular partners that drive their effects. Here, we present a molecular dynamics (MD) adaptive sampling algorithm, namely multiple walker supervised molecular dynamics (mwSuMD), to address complex structural transitions involving GPCRs without energy input. We first report the binding and unbinding of the vasopressin peptide from its receptor V2. Successively, we present the complete transition of the glucagon-like peptide-1 receptor (GLP-1R) from inactive to active, agonist and Gs-bound state, and the guanosine diphosphate (GDP) release from Gs. To our knowledge, this is the first time the whole sequence of events leading from an inactive GPCR to the GDP release is simulated without any energy bias. We demonstrate that mwSuMD can address complex binding processes intrinsically linked to protein dynamics out of reach of classic MD.

Prolonged signaling of backbone-modified glucagon-like peptide- analogues with diverse receptor trafficking

Signal duration and subcellular location are emerging as important facets of G protein-coupled receptor (GPCR) function. The glucagon-like peptide-1 receptor (GLP-1R), a clinically relevant class B1 GPCR, stimulates production of the second messenger cyclic adenosine monophosphate (cAMP) upon activation by the native hormone, GLP-1. cAMP production continues after the hormone-receptor complex has been internalized via endocytosis. Here, we report GLP-1 analogues that induce prolonged signaling relative to GLP-1. A single β-amino acid substitution at position 18, with the residue derived from (S,S)-trans-2-aminocyclopentanecarboxylic acid (ACPC), enhances signaling duration with retention of receptor endocytosis. Pairing ACPC at position 18 with a second substitution, α-aminoisobutyric acid (Aib) at position 16, abrogates endocytosis, but prolonged signaling is maintained. Prolonged signaling is sensitive to the structure of the β residue at position 18. Cryoelectron microscopy structures of two GLP-1 analogues bound to the GLP-1R:Gs complex suggest substantial alterations to bound peptide structure and dynamics compared to the GLP-1:GLP-1R:Gs complex. These structural findings strengthen an emerging view that agonist dynamics in the receptor-bound state influence signaling profiles. Our results advance understanding of the structural underpinnings of receptor activation and introduce tools for exploring the impact of spatiotemporal signaling profiles following GLP-1R activation.

Gestational diabetes mellitus-derived miR-7-19488 targets PIK3R2 mRNA to stimulate the abnormal development and maturation of offspring-islets

AIMS

Gestational diabetes mellitus (GDM) provides offspring with a hyper-metabolic intrauterine microenvironment. In this study, we aimed to identify key differential microRNAs in GDM-derived exosomes and explore the potential mechanisms of abnormal embryonic development of islets in offspring.

MAIN METHODS

Exosomes were extracted from umbilical vein blood of GDM and non-GDM (NGDM) parturients for microRNA sequencing. Offspring islets were collected on E18.5 and P0 to detect the expression and location of key proteins by immunofluorescence. Target binding of miR-7-19488 and PIK3R2 mRNA was verified using a dual-luciferase reporter assay. The miR-7-19488-mimic, PI3K/mTOR inhibitors were used to treat primarily islet cells to explore the relationship among miR-7-19488, PI3K, and Akt-FoxO1/mTORC1 signaling. The miR-7-19488 agomir was synthesized for further in vivo validation.

KEY FINDINGS

GDM-derived exosomes caused the overdevelopment of offspring-islets at E18.5 with an increased production of insulin and glucagon co-staining cells, increased number of α cells synthesizing GLP-1, and stimulation of mTORC1 singling, which were more serious at birth. The up-regulated miR-7-19488 in GDM-exosomes targeted PIK3R2 mRNA, leading to translation stagnation of p85β and activation of PI3K-Akt singling in fetal islets. Importantly, the activated PI3K-Akt-FoxO1 singling promoted development and differentiation of α and β cells and enhanced the GLP-1/GLP-1R axis, which cooperates with miR-7-19488 to activate PI3K-Akt-FoxO1/mTORC1 signaling, leading to the early initiation of the functional maturation of overdeveloped β cells.

SIGNIFICANCE

miR-7-19488 loaded in GDM-derived exosomes induce the abnormal overdevelopment and functional maturation of fetal islets, which is one of the contributors of high incidence of diabetes in adulthood.

In vivo functional profiling and structural characterisation of the human Glp1r A316T variant

Glucagon-like peptide-1 receptor agonists (GLP-1RAs) are effective therapies for type 2 diabetes (T2D) and obesity, yet patient responses are variable. Variation in the human Glp1r gene might be directly linked to therapeutic responses. A naturally occurring missense variant, A316T, protects against T2D and cardiovascular disease. Here, we have generated and characterised a human Glp1r A316T mouse model. Human Glp1r A316T/A316T mice displayed lower fasting blood glucose versus wildtype littermates, even under metabolic stress, and exhibited alterations in islet cytoarchitecture and α/β identity under a high-fat, high-sucrose diet. This was however associated with blunted responses to GLP-1RAs in vivo. Further investigations in rodent and human β-cell models demonstrated that human Glp1r A316T exhibits characteristics of constitutive activation but dampened GLP-1RA responses. Results are further supported by cryo-EM analyses and molecular dynamics simulations of GLP-1R A316T structure, collectively demonstrating that the A316T variant governs basal GLP-1R activity and pharmacological responses to GLP-1R-targeting therapies.

Class B1 GPCRs: insights into multireceptor pharmacology for the treatment of metabolic disease

The secretin-like, class B1 subfamily of seven transmembrane-spanning G protein-coupled receptors (GPCRs) consists of 15 members that coordinate important physiological processes. These receptors bind peptide ligands and use a distinct mechanism of activation that is driven by evolutionarily conserved structural features. For the class B1 receptors, the C-terminus of the cognate ligand is initially recognized by the receptor via an N-terminal extracellular domain that forms a hydrophobic ligand-binding groove. This binding enables the N-terminus of the ligand to engage deep into a large volume, open transmembrane pocket of the receptor. Importantly, the phylogenetic basis of this ligand-receptor activation mechanism has provided opportunities to engineer analogs of several class B1 ligands for therapeutic use. Among the most accepted of these are drugs targeting the glucagon-like peptide-1 (GLP-1) receptor for the treatment of type 2 diabetes and obesity. Recently, multifunctional agonists possessing activity at the GLP-1 receptor and the glucose-dependent insulinotropic polypeptide (GIP) receptor, such as tirzepatide, and others that also contain glucagon receptor activity, have been developed. In this article, we review members of the class B1 GPCR family with focus on receptors for GLP-1, GIP, and glucagon, including their signal transduction and receptor trafficking characteristics. The metabolic importance of these receptors is also highlighted, along with the benefit of polypharmacologic ligands. Furthermore, key structural features and comparative analyses of high-resolution cryogenic electron microscopy structures for these receptors in active-state complexes with either native ligands or multifunctional agonists are provided, supporting the pharmacological basis of such therapeutic agents.

Insights into the mechanism of crotamine and potential targets involved in obesity-related metabolic pathways

Crotamine (Ctm) is a peptide isolated from Crotalus durissus terrificus venom. This molecule has been demonstrated to diminish body weight gain and enhance browning in adipose tissue, glucose tolerance, and insulin sensitivity; hence, it has been postulated as an anti-obesogenic peptide. However, the mechanism to elicit the anti-obesogenic effects has yet to be elucidated. Thus, we investigated the possible interaction of Ctm with receptors involved in obesity-related metabolic pathways through protein-protein docking and molecular dynamics refinement. To test the anti-obesogenic mechanism of Ctm, we selected and retrieved 18 targets involved in obesity-related drug discovery from Protein Data Bank. Then, we performed protein-protein dockings. The best three Ctm-target models were selected and refined by molecular dynamics simulations. Molecular docking demonstrated that Ctm was able to interact with 13 of the 18 targets tested. Having a better docking score with glucagon-like peptide-1 receptor (GLP-1R) (-1430.2 kcal/mol), DPP-IV (dipeptidyl peptidase-IV) (-1781.7 kcal/mol) and α-glucosidase (-1232.3 kcal/mol). These three models were refined by molecular dynamics. Ctm demonstrated a higher affinity for GLP-1R (ΔG: -41.886 ± 2.289 kcal/mol). However, Ctm interaction was more stable with DPP-IV (RMSD: 0.360 ± 0.015 nm, Radius of gyration: 2.781 ± 0.009 nm). Moreover, the number of interactions and the molecular mechanics energies of Ctm residues suggest that the interaction of Ctm with these receptors is mainly mediated by basic-hydrophobic dyads Y1-K2, W31-R32, and W33-R34. Together, all these results allow elucidating a possible molecular mechanism behind the previously described anti-obesogenic effects.

Influence of Trp-Cage on the Function and Stability of GLP-1R Agonist Exenatide Derivatives

Exenatide (Ex4), a GLP-1 incretin mimetic polypeptide, is an effective therapeutic agent against diabetes and obesity. We highlight the indirect role of Ex4's structure-stabilizing Trp-cage (Tc) motif in governing GLP-1 receptor (GLP-1R) signal transduction. We use various Ex4 derivatives to explore how Tc compactness influences thermal stability, aggregation, enhancement of insulin secretion, and GLP-1R binding. We found that Ex4 variants decorated with fortified Tc motifs exhibit increased resistance to unfolding and aggregation but show an inverse relationship between the bioactivity and stability. Molecular dynamics simulations coupled with a rigid-body segmentation protocol to analyze dynamic interconnectedness revealed that the constrained Tc motifs remain intact within the receptor-ligand complexes but interfere with one of the major stabilizing contacts and recognition loci on the extracellular side of GLP-1R, dislodging the N-terminal activating region of the hormone mimetics, and restrict the free movement of TM6, the main signal transduction device of GLP-1R.

The G Protein-First Mechanism for Activation of the Class B Glucagon-like Peptide 1 Receptor Coupled to N-Terminal Domain-Mediated Conformational Progression

Recently, there has been a great deal of excitement about new glucagon-like peptide 1 receptor (GLP-1R) agonists (e.g., semaglutide and tirzepatide) that have received FDA approval for type 2 diabetes and obesity. Although effective, these drugs come with side effects that limit their use. While research efforts continue to focus intensively on long-lasting, orally administered GLP-1R medications with fewer side effects, a major impediment to developing improved GLP-1R medications is that the mechanism by which an agonist activates GLP-1R to imitate signaling is not known. Here we present and validate the G protein (GP)-first mechanism for the GLP-1R supported by extensive atomistic simulations. We propose that GLP-1R is preactivated through the formation of a GLP-1R-GP precoupled complex at the cell membrane prior to ligand binding. Despite a transmembrane helix 6 (TM6)-bentout conformation characteristic of activated GLP-1R, this precoupled complex remains unactivated until an agonist binds to elicit signaling. Notably, this new hypothesis offers a unified and predictive model for the activities of a series of full and partial agonists, including the peptides ExP5, GLP-1(7-36), and GLP-1(9-36). Most surprisingly, our simulations reveal an N-terminus domain (NTD)-swing/agonist-insertion mechanism wherein the long extracellular NTD of GLP-1R tightly holds the C-terminal half of the peptide agonist and progressively shifts the N-terminal head of the peptide to facilitate insertion into the orthosteric pocket. Our findings provide novel mechanistic insights into the activation and function of class B GPCRs and should provide a realistic basis for structure-based ligand design.

Glucagon-like peptide-1 receptor: mechanisms and advances in therapy

The glucagon-like peptide-1 (GLP-1) receptor, known as GLP-1R, is a vital component of the G protein-coupled receptor (GPCR) family and is found primarily on the surfaces of various cell types within the human body. This receptor specifically interacts with GLP-1, a key hormone that plays an integral role in regulating blood glucose levels, lipid metabolism, and several other crucial biological functions. In recent years, GLP-1 medications have become a focal point in the medical community due to their innovative treatment mechanisms, significant therapeutic efficacy, and broad development prospects. This article thoroughly traces the developmental milestones of GLP-1 drugs, from their initial discovery to their clinical application, detailing the evolution of diverse GLP-1 medications along with their distinct pharmacological properties. Additionally, this paper explores the potential applications of GLP-1 receptor agonists (GLP-1RAs) in fields such as neuroprotection, anti-infection measures, the reduction of various types of inflammation, and the enhancement of cardiovascular function. It provides an in-depth assessment of the effectiveness of GLP-1RAs across multiple body systems-including the nervous, cardiovascular, musculoskeletal, and digestive systems. This includes integrating the latest clinical trial data and delving into potential signaling pathways and pharmacological mechanisms. The primary goal of this article is to emphasize the extensive benefits of using GLP-1RAs in treating a broad spectrum of diseases, such as obesity, cardiovascular diseases, non-alcoholic fatty liver disease (NAFLD), neurodegenerative diseases, musculoskeletal inflammation, and various forms of cancer. The ongoing development of new indications for GLP-1 drugs offers promising prospects for further expanding therapeutic interventions, showcasing their significant potential in the medical field.

Positive allosteric modulation of a GPCR ternary complex

The activation of a G protein-coupled receptor (GPCR) leads to the formation of a ternary complex between agonist, receptor, and G protein that is characterized by high-affinity binding. Allosteric modulators bind to a distinct binding site from the orthosteric agonist and can modulate both the affinity and the efficacy of orthosteric agonists. The influence allosteric modulators have on the high-affinity active state of the GPCR-G protein ternary complex is unknown due to limitations on attempting to characterize this interaction in recombinant whole cell or membrane-based assays. Here, we use the purified M2 muscarinic acetylcholine receptor reconstituted into nanodiscs to show that, once the agonist-bound high-affinity state is promoted by the G protein, positive allosteric modulators stabilize the ternary complex that, in the presence of nucleotides, leads to an enhanced initial rate of signaling. Our results enhance our understanding of how allosteric modulators influence orthosteric ligand signaling and will aid the design of allosteric therapeutics.

Insights into the structure and activation mechanism of some class B1 GPCR family members

In humans, 15 genes encode the class B1 family of GPCRs, which are polypeptide hormone receptors characterized by having a large N-terminal extracellular domain (ECD) and receive signals from outside the cell to activate cellular response. For example, the insulinotropic polypeptide (GIP) stimulates the glucose-dependent insulinotropic polypeptide receptor (GIPR), while the glucagon receptor (GCGR) responds to glucagon by increasing blood glucose levels and promoting the breakdown of liver glycogen to induce the production of insulin. The glucagon-like peptides 1 and 2 (GLP-1 and GLP-2) elicit a response from glucagon-like peptide receptor types 1 and 2 (GLP1R and GLP2R), respectively. Since these receptors are implicated in the pathogenesis of diabetes, studying their activation is crucial for the development of effective therapies for the condition. With more structural information being revealed by experimental methods such as X-ray crystallography, cryo-EM, and NMR, the activation mechanism of class B1 GPCRs becomes unraveled. The available crystal and cryo-EM structures reveal that class B1 GPCRs follow a two-step model for peptide binding and receptor activation. The regions close to the C-termini of hormones interact with the N-terminal ECD of the receptor while the regions close to the N-terminus of the peptide interact with the TM domain and transmit signals. This review highlights the structural details of class B1 GPCRs and their conformational changes following activation. The roles of MD simulation in characterizing those conformational changes are briefly discussed, providing insights into the potential structural exploration for future ligand designs.

GLP1-GIP receptor co-agonists: a promising evolution in the treatment of type 2 diabetes

Type 2 diabetes represents a growing challenge for global public health. Its prevalence is increasing worldwide, and, like obesity, it affects progressively younger populations compared to the past, with potentially greater impact on chronic complications. Dual glucagon like peptide 1 (GLP1) and glucose-dependent insulinotropic peptide (GIP) receptor agonists are among the new pharmacological strategies recently developed to address this challenge. Tirzepatide, characterized by its ability to selectively bind and activate receptors for the intestinal hormones GIP and GLP-1, has been tested in numerous clinical studies and is already currently authorized in several countries for the treatment of type 2 diabetes and obesity. In this context, the aim of the present document is to summarize, in the form of a narrative literature review, the currently available data on the main mechanisms of action of GIP/GLP-1 co-agonists and the clinical effects of tirzepatide evaluated in various clinical trials.

The impact of GLP-1 signalling on the energy metabolism of pancreatic islet β-cells and extrapancreatic tissues

Glucagon-like peptide-1 signalling impacts glucose homeostasis and appetite thereby indirectly affecting substrate availability at the whole-body level. The incretin canonically produces an insulinotropic effect, thereby lowering blood glucose levels by promoting the uptake and inhibiting the production of the sugar by peripheral tissues. Likewise, GLP-1 signalling within the central nervous system reduces the appetite and food intake, whereas its gastric effect delays the absorption of nutrients, thus improving glycaemic control and reducing the risk of postprandial hyperglycaemia. We review the molecular aspects of the GLP-1 signalling, focusing on its impact on intracellular energy metabolism. Whilst the incretin exerts its effects predominantly via a Gs receptor, which decodes the incretin signal into the elevation of intracellular cAMP levels, the downstream signalling cascades within the cell, acting on fast and slow timescales, resulting in an enhancement or an attenuation of glucose catabolism, respectively.

Cholesterol-dependent dynamic changes in the conformation of the type 1 cholecystokinin receptor affect ligand binding and G protein coupling

Development of optimal therapeutics for disease states that can be associated with increased membrane cholesterol requires better molecular understanding of lipid modulation of the drug target. Type 1 cholecystokinin receptor (CCK1R) agonist actions are affected by increased membrane cholesterol, enhancing ligand binding and reducing calcium signaling, while agonist actions of the closely related CCK2R are not. In this work, we identified a set of chimeric human CCK1R/CCK2R mutations that exchange the cholesterol sensitivity of these 2 receptors, providing powerful tools when expressed in CHO and HEK-293 model cell lines to explore mechanisms. Static, low energy, high-resolution structures of the mutant CCK1R constructs, stabilized in complex with G protein, were not substantially different, suggesting that alterations to receptor dynamics were key to altered function. We reveal that cholesterol-dependent dynamic changes in the conformation of the helical bundle of CCK receptors affects both ligand binding at the extracellular surface and G protein coupling at the cytosolic surface, as well as their interrelationships involved in stimulus-response coupling. This provides an ideal setting for potential allosteric modulators to correct the negative impact of membrane cholesterol on CCK1R.

Building the Glucagon-Like Peptide-1 Receptor Brick by Brick: Revisiting a 1993 Diabetes Classic by Thorens et al

The glucagon-like peptide-1 receptor (GLP-1R) is a class B G protein-coupled receptor involved in the regulation of blood glucose levels and food intake. Stabilized agonists targeting GLP-1R are used in the treatment of type 2 diabetes and have recently become a breakthrough obesity therapy. Here, we revisit a classic article in Diabetes by Thorens et al. that described the cloning, sequencing, and functional expression of the human GLP-1R. The article also demonstrated that exendin4(1-39) was a full agonist of the human GLP-1R whereas exendin4(9-39) was a full antagonist. We discuss how the knowledge imparted by these studies has gone on to inform multiple strands of GLP-1R biology over the past three decades, including pharmacology, signaling, human genetics, structural biology, and chemical biology.

GIP-derived GIP receptor antagonists - a review of their role in GIP receptor pharmacology

Surprisingly, agonists, as well as antagonists of the glucose-dependent insulinotropic polypeptide receptor (GIPR), are currently being used or investigated as treatment options for type 2 diabetes and obesity - and both, when combined with glucagon-like peptide 1 receptor (GLP-1R) agonism, enhance GLP-1-induced glycemia and weight loss further. This paradox raises several questions regarding not only the mechanisms of actions of GIP but also the processes engaged during the activation of both the GIP and GLP-1 receptors. Here, we provide an overview of studies of the properties and actions of peptide-derived GIPR antagonists, focusing on GIP(3-30)NH2, a naturally occurring N- and C-terminal truncation of GIP(1-42). GIP(3-30)NH2 was the first GIPR antagonist administered to humans. GIP(3-30)NH2 and a few additional antagonists, like Pro3-GIP, have been used in both in vitro and in vivo studies to elucidate the molecular and cellular consequences of GIPR inhibition, desensitization, and internalization and, at a larger scale, the role of the GIP system in health and disease. We provide an overview of these studies combined with recent knowledge regarding the effects of naturally occurring variants of the GIPR system and species differences within the GIP system to enhance our understanding of the GIPR as a drug target.

Distinct roles of the extracellular surface residues of glucagon-like peptide-1 receptor in β-arrestin 1/2 signaling

Glucagon-like peptide-1 receptor (GLP-1R) is a prime drug target for type 2 diabetes and obesity. The ligand initiated GLP-1R interaction with G protein has been well studied, but not with β-arrestin 1/2. Therefore, bioluminescence resonance energy transfer (BRET), mutagenesis and an operational model were used to evaluate the roles of 85 extracellular surface residues on GLP-1R in β-arrestin 1/2 recruitment triggered by three representative GLP-1R agonists (GLP-1, exendin-4 and oxyntomodulin). Residues selectively regulated β-arrestin 1/2 recruitment for diverse ligands, and β-arrestin isoforms were identified. Mutation of residues K130-S136, L142 and Y145 on the transmembrane helix 1 (TM1)-extracellular domain (ECD) linker decreased β-arrestin 1 recruitment but increased β-arrestin 2 recruitment. Other extracellular loop (ECL) mutations, including P137A, Q211A, D222A and M303A selectively affected β-arrestin 1 recruitment while D215A, L217A, Q221A, S223A, Y289A, S301A, F381A and I382A involved more in β-arrestin 2 recruitment for the ligands. Oxyntomodulin engaged more broadly with GLP-1R extracellular surface to drive β-arrestin 1/2 recruitment than GLP-1 and exendin-4; I147, W214 and L218 involved in β-arrestin 1 recruitment, while L141, D215, L218, D293 and F381 in β-arrestin 2 recruitment for oxyntomodulin particularly. Additionally, the non-conserved residues on β-arrestin 1/2 C-domains contributed to interaction with GLP-1R. Further proteomic profiling of GLP-1R stably expressed cell line upon ligand stimulation with or without β-arrestin 1/2 overexpression demonstrated both commonly and biasedly regulated proteins and pathways associated with cognate ligands and β-arrestins. Our study offers valuable information about ligand induced β-arrestin recruitment mediated by GLP-1R and consequent intracellular signaling events.

Human gestational diabetes mellitus-derived exosomes impair glucose homeostasis in pregnant mice and stimulate functional maturation of offspring-islets

AIMS

Pancreatic islets undergo critical development and functional maturation during the perinatal period when they are highly sensitive to microenvironment. We aim to determine the effects and mechanisms of gestational diabetes mellitus (GDM) hypermetabolic stress on glucose homeostasis in pregnant mice and functional maturation of the islets of their offspring.

MAIN METHODS

Exosomes were extracted from the umbilical vein blood of individuals with or without GDM for administration to pregnant mice. The blood glucose, serum insulin, glycosylated hemoglobin, and lipopolysaccharide levels were measured in pregnant mice. The expression and localization of insulin, glucagon, PC1/3, PDX1, and p-S6 in the islets of neonatal rats were continuously monitored using immunofluorescence to evaluate their functional status. Primary islet cells were cultured and treated with GDM exosomes and exendin to determine the expression of GLP-1R, AKT, p-AKT, and p-S6 via western blotting.

KEY FINDINGS

GDM exosomes induced remarkable oral glucose intolerance, hyperinsulinemia, and metabolic inflammation in pregnant mice. The islets of GDM offspring exhibited high insulin, glucagon, PC1/3, PDX1, and p-S6 expression at and after birth, and activation of the local GLP-1/GLP-1R axis. The functional maturation of normal-offspring islets did not commence until after birth, while it was activated prior to birth in GDM offspring, seriously disrupting the whole process. GDM exosomes activated the GLP-1/GLP-1R axis between α and β cells, and stimulated functional maturation of β cells via the Akt-mTORC1-pS6 pathway.

SIGNIFICANCE

These findings provide preliminary insights into the mechanisms underlying the high incidence of diabetes in the offspring of mothers with GDM.

PACAP key interactions with PAC1, VPAC1, and VPAC2 identified by molecular dynamics simulations

The neuropeptide pituitary adenylate cyclase-activating polypeptide (PACAP) belongs to the glucagon/secretin family. PACAP interacts with the pituitary adenylate cyclase-activating polypeptide receptor type 1 (PAC1) and vasoactive intestinal peptide receptors 1 and 2 (VPAC1 and VPAC2), exhibiting functions in the immune, endocrine, and nervous systems. This peptide is upregulated in numerous instances of brain injury, acting as a neuroprotective agent. It can also suppress HIV-1 and SARS-CoV-2 viral replication in vitro. This work aimed to identify, in each peptide-receptor system, the most relevant residues for complex stability and interaction energy communication via Molecular Dynamics (MD), Free Energy calculations, and Protein-energy networks, thus revealing in detail the underlying mechanisms of activation of these receptors. Hydrogen bond formation, interaction energies, and computational alanine scanning between PACAP and its receptors showed that His1, Asp3, Arg12, Arg14, and Lys15 are crucial to the peptide's stability. Furthermore, several PACAP interactions with structurally conserved positions deemed necessary in GPCR B1 activation, including Arg2.60, Lys2.67, and Glu7.42, were significant for the peptide's stability within the receptors. According to the protein-energy network, the connection between Asp3 of PACAP and the receptors' conserved Arg2.60 represents a critical energy communication hub in all complexes. Additionally, the ECDs of the receptors were also found to function as energy communication hubs for PACAP. Although the overall binding mode of PACAP in the three receptors was found to be highly conserved, Arg12 and Tyr13 of PACAP were more prominent in complex with PAC1, while Ser2 of PACAP was with VPAC2. The detailed analyses performed in this work pave the way for using PACAP and its receptors as therapeutic targets.Communicated by Ramaswamy H. Sarma.

Recent advances in the exploration and discovery of SARS-CoV-2 inhibitory peptides from edible animal proteins

The severe acute respiratory syndrome coronavirus type 2 (SARS-CoV-2), which causes the coronavirus disease 2019 (COVID-19), is spreading worldwide. Although the COVID-19 epidemic has passed its peak of transmission, the harm it has caused deserves our attention. Scientists are striving to develop medications that can effectively treat COVID-19 symptoms without causing any adverse reactions. SARS-CoV-2 inhibitory peptides derived from animal proteins have a wide range of functional activities in addition to safety. Identifying animal protein sources is crucial to obtaining SARS-CoV-2 inhibitory peptides from animal sources. This review aims to reveal the mechanisms of action of these peptides on SARS-CoV-2 and the possibility of animal proteins as a material source of SARS-CoV-2 inhibitory peptides. Also, it introduces the utilization of computer-aided design methods, phage display, and drug delivery strategies in the research on SARS-CoV-2 inhibitor peptides from animal proteins. In order to identify new antiviral peptides and boost their efficiency, we recommend investigating the interaction between SARS-CoV-2 inhibitory peptides from animal protein sources and non-structural proteins (Nsps) using a variety of technologies, including computer-aided drug approaches, phage display techniques, and drug delivery techniques. This article provides useful information for the development of novel anti-COVID-19 drugs.

Palmitoylation of the Glucagon-like Peptide-1 Receptor Modulates Cholesterol Interactions at the Receptor-Lipid Microenvironment

The G protein-coupled receptor (GPCR) superfamily of cell surface receptors has been shown to be functionally modulated by post-translational modifications. The glucagon-like peptide receptor-1 (GLP-1R), which is a drug target in diabetes and obesity, undergoes agonist-dependent palmitoyl tail conjugation. The palmitoylation in the C-terminal domain of GLP-1R has been suggested to modulate the receptor-lipid microenvironment. In this work, we have performed coarse-grain molecular dynamics simulations of palmitoylated and nonpalmitoylated GLP-1R to analyze the differential receptor-lipid interactions. Interestingly, the placement and dynamics of the C-terminal domain of GLP-1R are found to be directly dependent on the palmitoyl tail. We observe that both cholesterol and phospholipids interact with the receptor but display differential interactions in the presence and absence of the palmitoyl tail. We characterize important cholesterol-binding sites and validate sites that have been previously reported in experimentally resolved structures of the receptor. We show that the receptor acts like a conduit for cholesterol flip-flop by stabilizing cholesterol in the membrane core. Taken together, our work represents an important step in understanding the molecular effects of lipid modifications in GPCRs.

Effects of liraglutide on ANP secretion and cardiac dynamics

To observe the effects of liraglutide (analog of glucagon-like peptide 1 (GLP-1)) on atrial natriuretic peptide (ANP) secretion and atrial dynamics, an ex vivo isolated rat atrial perfusion model was used to determine atrial ANP secretion and pulse pressure. DPP-4-/- mice were also established in vivo. ANP levels were determined by radioimmunoassay; GLP-1 content was determined by Elisa. The expression levels of GLP-1 receptor (GLP-1R), PI3K/AKT/mTOR, piezo 1, and cathepsin K were analyzed by Western blot. In the clinical study, patients with acute coronary syndrome (ACS) had low levels of plasma GLP-1 but relatively high levels of plasma ANP. In ex vivo (3.2 nmol/L) and in vivo (30 μg/kg) models, liraglutide significantly decreased ANP levels and atrial pulse pressure. Exendin9-39 alone (GLP-1R antagonist) reversibly significantly increased ANP secretion, and the reduction effect of liraglutide on the secretion of ANP was significantly alleviated by Exendin9-39. Exendin9-39 demonstrated slightly decreased atrial pulse pressure; however, combined liraglutide and Exendin9-39 significantly decreased atrial pulse pressure. Ly294002 (PI3K/AKT inhibitor) inhibited the increase of ANP secretion by liraglutide for a short time, while Ly294002 didn't counteract the decrease in pulse pressure by liraglutide in atrial dynamics studies. Liraglutide increased the expression of GLP-1R and PI3K/AKT/mTOR in isolated rat atria and the hearts of mice in vivo, whereas Exendin9-39 reversibly reduced the expression of GLP-1R and PI3K/AKT/mTOR. Piezo 1 was significantly decreased in wild type and DPP-4-/- mouse heart or isolated rat atria after being treated with liraglutide. Cathepsin K expression was only decreased in in vivo model hearts. Liraglutide can inhibit ANP secretion while decreasing atrial pulse pressure mediated by GLP-1R. Liraglutide probably plays a role in the reduction of ANP secretion via the PI3K/AKT/mTOR signaling pathway. Piezo 1 and cathepsin K may be involved in the liraglutide mechanism of reduction.

GLP-1R signaling neighborhoods associate with the susceptibility to adverse drug reactions of incretin mimetics

G protein-coupled receptors are important drug targets that engage and activate signaling transducers in multiple cellular compartments. Delineating therapeutic signaling from signaling associated with adverse events is an important step towards rational drug design. The glucagon-like peptide-1 receptor (GLP-1R) is a validated target for the treatment of diabetes and obesity, but drugs that target this receptor are a frequent cause of adverse events. Using recently developed biosensors, we explored the ability of GLP-1R to activate 15 pathways in 4 cellular compartments and demonstrate that modifications aimed at improving the therapeutic potential of GLP-1R agonists greatly influence compound efficacy, potency, and safety in a pathway- and compartment-selective manner. These findings, together with comparative structure analysis, time-lapse microscopy, and phosphoproteomics, reveal unique signaling signatures for GLP-1R agonists at the level of receptor conformation, functional selectivity, and location bias, thus associating signaling neighborhoods with functionally distinct cellular outcomes and clinical consequences.

An inverse agonist of orphan receptor GPR61 acts by a G protein-competitive allosteric mechanism

GPR61 is an orphan GPCR related to biogenic amine receptors. Its association with phenotypes relating to appetite makes it of interest as a druggable target to treat disorders of metabolism and body weight, such as obesity and cachexia. To date, the lack of structural information or a known biological ligand or tool compound has hindered comprehensive efforts to study GPR61 structure and function. Here, we report a structural characterization of GPR61, in both its active-like complex with heterotrimeric G protein and in its inactive state. Moreover, we report the discovery of a potent and selective small-molecule inverse agonist against GPR61 and structural elucidation of its allosteric binding site and mode of action. These findings offer mechanistic insights into an orphan GPCR while providing both a structural framework and tool compound to support further studies of GPR61 function and modulation.

Structure and Molecular Mechanism of Signaling for the Glucagon-like Peptide-1 Receptor Bound to Gs Protein and Exendin-P5 Biased Agonist

The glucagon-like peptide-1 receptor (GLP-1R) is a key regulator of blood glucose and a prime target for the treatment of type II diabetes and obesity with multiple public drugs. Here we present a comprehensive computational analysis of the interactions of the activated GLP-1R-Gs signaling complex with a G protein biased agonist, Exendin P5 (ExP5), which possesses a unique N-terminal sequence responsible for the signal bias. Using a refined all-atom model of the ExP5-GLP-1R-Gs complex in molecular dynamics (MD) simulations, we propose a novel mechanism of conformation transduction in which the unique interaction network of ExP5 N-terminus propagates the binding signal across an array of conserved residues at the transmembrane domain to enhance Gs protein coupling at the cytoplasmic end of the receptor. Our simulations reveal previously unobserved interactions important for activation by ExP5 toward GDP-GTP signaling, providing new insights into the mechanism of class B G protein-coupled receptor (GPCR) signaling. These findings offer a framework for the structure-based design of more effective therapeutics.

Hormone Analogues with Unique Signaling Profiles from Replacement of α-Residue Triads with β/γ Diads

We have applied an underexplored backbone modification strategy to generate new analogues of peptides that activate two clinically important class B1 G protein-coupled receptors (GPCRs). Most peptide modification strategies involve changing side chains or, less commonly, changing the configuration at side chain-bearing carbons (i.e., l residues replaced by d residues). In contrast, backbone modifications alter the number of backbone atoms and the identities of backbone atoms relative to a poly-α-amino acid backbone. Starting from the peptide agonists PTH(1-34) (the first 34 residues of the parathyroid hormone, used clinically as the drug teriparatide) and glucagon-like peptide-1 (7-36) (GLP-1(7-36)), we replaced native α-residue triads with a diad composed of a β-amino acid residue and a γ-amino acid residue. The β/γ diad retains the number of backbone atoms in the ααα triad. Because the β and γ residue each bear a single side chain, we implemented ααα→βγ replacements at sites that contained a Gly residue (i.e., at α-residue triads that presented only two side chains). All seven of the α/β/γ-peptides derived from PTH(1-34) or GLP-1(7-36) bind to the cognate receptor (the PTHR1 or the GLP-1R), but they vary considerably in their activity profiles. Outcomes include functional mimicry of the all-α agonist, receptor-selective agonist activity, biased agonism, or strong binding with weak activation, which could lead to antagonist development. Collectively, these findings demonstrate that ααα→βγ replacements, which are easily implemented via solid-phase synthesis, can generate peptide hormone analogues that display unique and potentially useful signaling behavior.

GWAS of random glucose in 476,326 individuals provide insights into diabetes pathophysiology, complications and treatment stratification

Conventional measurements of fasting and postprandial blood glucose levels investigated in genome-wide association studies (GWAS) cannot capture the effects of DNA variability on 'around the clock' glucoregulatory processes. Here we show that GWAS meta-analysis of glucose measurements under nonstandardized conditions (random glucose (RG)) in 476,326 individuals of diverse ancestries and without diabetes enables locus discovery and innovative pathophysiological observations. We discovered 120 RG loci represented by 150 distinct signals, including 13 with sex-dimorphic effects, two cross-ancestry and seven rare frequency signals. Of these, 44 loci are new for glycemic traits. Regulatory, glycosylation and metagenomic annotations highlight ileum and colon tissues, indicating an underappreciated role of the gastrointestinal tract in controlling blood glucose. Functional follow-up and molecular dynamics simulations of lower frequency coding variants in glucagon-like peptide-1 receptor (GLP1R), a type 2 diabetes treatment target, reveal that optimal selection of GLP-1R agonist therapy will benefit from tailored genetic stratification. We also provide evidence from Mendelian randomization that lung function is modulated by blood glucose and that pulmonary dysfunction is a diabetes complication. Our investigation yields new insights into the biology of glucose regulation, diabetes complications and pathways for treatment stratification.

Structural analysis of the dual agonism at GLP-1R and GCGR

Glucagon-like peptide-1 receptor (GLP-1R) and glucagon receptor (GCGR), two members of class B1 G protein-coupled receptors, play important roles in glucose homeostasis and energy metabolism. They share a high degree of sequence homology but have different functionalities. Unimolecular dual agonists of both receptors developed recently displayed better clinical efficacies than that of monotherapy. To study the underlying molecular mechanisms, we determined high-resolution cryo-electron microscopy structures of GLP-1R or GCGR in complex with heterotrimeric Gs protein and three GLP-1R/GCGR dual agonists including peptide 15, MEDI0382 (cotadutide) and SAR425899 with variable activating profiles at GLP-1R versus GCGR. Compared with related structures reported previously and supported by our published pharmacological data, key residues responsible for ligand recognition and dual agonism were identified. Analyses of peptide conformational features revealed a difference in side chain orientations within the first three residues, indicating that distinct engagements in the deep binding pocket are required to achieve receptor selectivity. The middle region recognizes extracellular loop 1 (ECL1), ECL2, and the top of transmembrane helix 1 (TM1) resulting in specific conformational changes of both ligand and receptor, especially the dual agonists reshaped ECL1 conformation of GLP-1R relative to that of GCGR, suggesting an important role of ECL1 interaction in executing dual agonism. Structural investigation of lipid modification showed a better interaction between lipid moiety of MEDI0382 and TM1-TM2 cleft, in line with its increased potency at GCGR than SAR425899. Together, the results provide insightful information for the design and development of improved therapeutics targeting these two receptors simultaneously.

Dynamics of the Second Extracellular Loop Control Transducer Coupling of Peptide-Activated GPCRs

Many peptide-activated rhodopsin-like GPCRs share a β-hairpin folding motif in the extracellular loop 2 (ECL2), which interacts with the peptide ligand while at the same time being connected to transmembrane helix 3 (TM3) via a highly conserved disulfide bond. Currently, it remains unknown whether the coupling of the specifically shaped ECL2 to TM3 influences the activation of peptide-activated GPCRs. We investigated this possibility in a selection of peptide GPCRs with known structures. Most of the receptors with cysteine to alanine mutations folded like the respective wild-type and resided in the cell membrane, challenging pure folding stabilization by the disulfide bridge. G-protein signaling of the disulfide mutants was retained to a greater extent in secretin-like GPCRs than in rhodopsin-like GPCRs, while recruitment of arrestin was completely abolished in both groups, which may be linked to alterations in ligand residence time. We found a correlation between receptor activity of the neuropeptide Y2 receptor and alterations in ECL2 dynamics using engineered disulfide bridges or site-directed spin labeling and EPR spectroscopy. These data highlight the functional importance of the TM3-ECL2 link for the activation of specific signaling pathways in peptide-activated GPCRs, which might have implications for future drug discovery.

Mapping Peptide-Protein Interactions by Amine-Reactive Cleavable Photoaffinity Reagents

Photoaffinity labeling followed by tandem mass spectrometry is an often used strategy to identify protein targets of small-molecule drugs or drug candidates, which, under ideal conditions, enables the identification of the actual drug binding site. In the case of bioactive peptides, however, identifying the distinct binding site is hampered because of complex fragmentation patterns during tandem mass spectrometry. We here report the development and use of small cleavable photoaffinity reagents that allow functionalization of bioactive peptides for light-induced covalent binding to their protein targets. Upon cleavage of the covalently linked peptide drug, a chemical remnant of a defined mass remains on the bound amino acid, which is then used to unambiguously identify the drug binding site. Applying our approach to known peptide-drug/protein pairs with reported crystal structures, such as the calmodulin-melittin interaction, we were able to validate the identified binding sites based on structural models. Overall, our cleavable photoaffinity labeling strategy represents a powerful tool to enable the identification of protein targets and specific binding sites of a wide variety of bioactive peptides in the future.

Selective polypeptide ligand binding to the extracellular surface of the transmembrane domains of the class B GPCRs GLP-1R and GCGR

Crystal and cryo-EM structures of the glucagon-like peptide-1 receptor (GLP-1R) and glucagon receptor (GCGR) bound with their peptide ligands have been obtained with full-length constructs, indicating that the extracellular domain (ECD) is indispensable for specific ligand binding. This article complements these data with studies of ligand recognition of the two receptors in solution. Paramagnetic NMR relaxation enhancement measurements using dual labeling with fluorine-19 probes on the receptor and nitroxide spin labels on the peptide ligands provided new insights. The glucagon-like peptide-1 (GLP-1) was found to interact with GLP-1R by selective binding to the extracellular surface. The ligand selectivity toward the extracellular surface of the receptor was preserved in the transmembrane domain (TMD) devoid of the ECD. The dual labeling approach further provided evidence of cross-reactivity of GLP-1R and GCGR with glucagon and GLP-1, respectively, which is of interest in the context of medical treatments using combinations of the two polypeptides.

Differential Responses of the GLP-1 and GLP-2 Receptors to N-Terminal Modification of a Dual Agonist

Class B1 G protein-coupled receptors (GPCRs), collectively, respond to a diverse repertoire of extracellular polypeptide agonists and transmit the encoded messages to cytosolic partners. To fulfill these tasks, these highly mobile receptors must interconvert among conformational states in response to agonists. We recently showed that conformational mobility in polypeptide agonists themselves plays a role in activation of one class B1 GPCR, the receptor for glucagon-like peptide-1 (GLP-1). Exchange between helical and nonhelical conformations near the N-termini of agonists bound to the GLP-1R was revealed to be critical for receptor activation. Here, we ask whether agonist conformational mobility plays a role in the activation of a related receptor, the GLP-2R. Using variants of the hormone GLP-2 and the designed clinical agonist glepaglutide (GLE), we find that the GLP-2R is quite tolerant of variations in α-helical propensity near the agonist N-terminus, which contrasts with signaling at the GLP-1R. A fully α-helical conformation of the bound agonist may be sufficient for GLP-2R signal transduction. GLE is a GLP-2R/GLP-1R dual agonist, and the GLE system therefore enables direct comparison of the responses of these two GPCRs to a single set of agonist variants. This comparison supports the conclusion that the GLP-1R and GLP-2R differ in their response to variations in helical propensity near the agonist N-terminus. The data offer a basis for development of new hormone analogues with distinctive and potentially useful activity profiles; for example, one of the GLE analogues is a potent agonist of the GLP-2R but also a potent antagonist of the GLP-1R, a novel form of polypharmacology.

Pharmacological hallmarks of allostery at the M4 muscarinic receptor elucidated through structure and dynamics

Allosteric modulation of G protein-coupled receptors (GPCRs) is a major paradigm in drug discovery. Despite decades of research, a molecular-level understanding of the general principles that govern the myriad pharmacological effects exerted by GPCR allosteric modulators remains limited. The M₄ muscarinic acetylcholine receptor (M₄ mAChR) is a validated and clinically relevant allosteric drug target for several major psychiatric and cognitive disorders. In this study, we rigorously quantified the affinity, efficacy, and magnitude of modulation of two different positive allosteric modulators, LY2033298 (LY298) and VU0467154 (VU154), combined with the endogenous agonist acetylcholine (ACh) or the high-affinity agonist iperoxo (Ipx), at the human M₄ mAChR. By determining the cryo-electron microscopy structures of the M₄ mAChR, bound to a cognate G_i1 protein and in complex with ACh, Ipx, LY298-Ipx, and VU154-Ipx, and applying molecular dynamics simulations, we determine key molecular mechanisms underlying allosteric pharmacology. In addition to delineating the contribution of spatially distinct binding sites on observed pharmacology, our findings also revealed a vital role for orthosteric and allosteric ligand-receptor-transducer complex stability, mediated by conformational dynamics between these sites, in the ultimate determination of affinity, efficacy, cooperativity, probe dependence, and species variability. There results provide a holistic framework for further GPCR mechanistic studies and can aid in the discovery and design of future allosteric drugs.

Conformational Dynamics of the Activated GLP-1 Receptor-Gs Complex Revealed by Cross-Linking Mass Spectrometry and Integrative Structure Modeling

Despite advances in characterizing the structures and functions of G protein-coupled receptors (GPCRs), our understanding of GPCR activation and signaling is still limited by the lack of information on conformational dynamics. It is particularly challenging to study the dynamics of GPCR complexes with their signaling partners because of their transient nature and low stability. Here, by combining cross-linking mass spectrometry (CLMS) with integrative structure modeling, we map the conformational ensemble of an activated GPCR-G protein complex at near-atomic resolution. The integrative structures describe heterogeneous conformations for a high number of potential alternative active states of the GLP-1 receptor-G_s complex. These structures show marked differences from the previously determined cryo-EM structure, especially at the receptor-G_s interface and in the interior of the G_s heterotrimer. Alanine-scanning mutagenesis coupled with pharmacological assays validates the functional significance of 24 interface residue contacts only observed in the integrative structures, yet absent in the cryo-EM structure. Through the integration of spatial connectivity data from CLMS with structure modeling, our study provides a new approach that is generalizable to characterizing the conformational dynamics of GPCR signaling complexes.

Molecular insights into peptide agonist engagement with the PTH receptor

The parathyroid hormone (PTH) 1 receptor (PTH1R) is a G protein-coupled receptor (GPCR) that regulates skeletal development and calcium homeostasis. Here, we describe cryo-EM structures of the PTH1R in complex with fragments of the two hormones, PTH and PTH-related protein, the drug abaloparatide, as well as the engineered tool compounds, long-acting PTH (LA-PTH) and the truncated peptide, M-PTH(1-14). We found that the critical N terminus of each agonist engages the transmembrane bundle in a topologically similar fashion, reflecting similarities in measures of Gαs activation. The full-length peptides induce subtly different extracellular domain (ECD) orientations relative to the transmembrane domain. In the structure bound to M-PTH, the ECD is unresolved, demonstrating that the ECD is highly dynamic when unconstrained by a peptide. High resolutions enabled identification of water molecules near peptide and G protein binding sites. Our results illuminate the action of orthosteric agonists of the PTH1R.

Effects of site-directed mutagenesis of GLP-1 and glucagon receptors on signal transduction activated by dual and triple agonists

The paradigm of one drug against multiple targets, known as unimolecular polypharmacology, offers the potential to improve efficacy while overcoming some adverse events associated with the treatment. This approach is best exemplified by targeting two or three class B1 G protein-coupled receptors, namely, glucagon-like peptide-1 receptor (GLP-1R), glucagon receptor (GCGR) and glucose-dependent insulinotropic polypeptide receptor for treatment of type 2 diabetes and obesity. Some of the dual and triple agonists have already shown initial successes in clinical trials, although the molecular mechanisms underlying their multiplexed pharmacology remain elusive. In this study we employed structure-based site-directed mutagenesis together with pharmacological assays to compare agonist efficacy across two key signaling pathways, cAMP accumulation and ERK1/2 phosphorylation (pERK1/2). Three dual agonists (peptide 15, MEDI0382 and SAR425899) and one triple agonist (peptide 20) were evaluated at GLP-1R and GCGR, relative to the native peptidic ligands (GLP-1 and glucagon). Our results reveal the existence of residue networks crucial for unimolecular agonist-mediated receptor activation and their distinct signaling patterns, which might be useful to the rational design of biased drug leads.

Structural Understanding of Peptide-Bound G Protein-Coupled Receptors: Peptide-Target Interactions

The activation of G protein-coupled receptors (GPCRs) is triggered by ligand binding to their orthosteric sites, which induces ligand-specific conformational changes. Agonists and antagonists bound to GPCR orthosteric sites provide detailed information on ligand-binding modes. Among these, peptide ligands play an instrumental role in GPCR pharmacology and have attracted increased attention as therapeutic drugs. The recent breakthrough in GPCR structural biology has resulted in the remarkable availability of peptide-bound GPCR complexes. Despite the several structural similarities shared by these receptors, they exhibit distinct features in terms of peptide recognition and receptor activation. From this perspective, we have summarized the current status of peptide-bound GPCR structural complexes, largely focusing on the interactions between the receptor and its peptide ligand at the orthosteric site. In-depth structural investigations have yielded valuable insights into the molecular mechanisms underlying peptide recognition. This study would contribute to the discovery of GPCR peptide drugs with improved therapeutic effects.

New insights into the structure and function of class B1 GPCRs

G protein-coupled receptors (GPCRs) are the largest family of cell surface receptors. Class B1 GPCRs constitute a subfamily of 15 receptors that characteristically contain large extracellular domains (ECDs) and respond to long polypeptide hormones. Class B1 GPCRs are critical regulators of homeostasis, and as such, many are important drug targets. While most transmembrane proteins, including GPCRs, are recalcitrant to crystallization, recent advances in electron cryo-microscopy (cryo-EM) have facilitated a rapid expansion of the structural understanding of membrane proteins. As a testament to this success, structures for all the class B1 receptors bound to G proteins have been determined by cryo-EM in the past five years. Further advances in cryo-EM have uncovered dynamics of these receptors, ligands, and signalling partners. Here, we examine the recent structural underpinnings of the class B1 GPCRs with an emphasis on structure-function relationships.

Targeting VIP and PACAP Receptor Signaling: New Insights into Designing Drugs for the PACAP Subfamily of Receptors

Pituitary Adenylate Cyclase-Activating Peptide (PACAP) and Vasoactive Intestinal Peptide (VIP) are neuropeptides involved in a diverse array of physiological and pathological processes through activating the PACAP subfamily of class B1 G protein-coupled receptors (GPCRs): VIP receptor 1 (VPAC1R), VIP receptor 2 (VPAC2R), and PACAP type I receptor (PAC1R). VIP and PACAP share nearly 70% amino acid sequence identity, while their receptors PAC1R, VPAC1R, and VPAC2R share 60% homology in the transmembrane regions of the receptor. PACAP binds with high affinity to all three receptors, while VIP binds with high affinity to VPAC1R and VPAC2R, and has a thousand-fold lower affinity for PAC1R compared to PACAP. Due to the wide distribution of VIP and PACAP receptors in the body, potential therapeutic applications of drugs targeting these receptors, as well as expected undesired side effects, are numerous. Designing selective therapeutics targeting these receptors remains challenging due to their structural similarities. This review discusses recent discoveries on the molecular mechanisms involved in the selectivity and signaling of the PACAP subfamily of receptors, and future considerations for therapeutic targeting.

In-silico study of the interactions between acylated glucagon like-peptide-1 analogues and the native receptor

We have performed a series of multiple molecular dynamics (MD) simulations of glucagon-like peptide-1 (GLP-1) and acylated GLP-1 analogues in complex with the endogenous receptor (GLP-1R) to obtain a molecular understanding of how fatty acid (FA) chain structure, acylation position on the peptide, and presence of a linker affect the binding. MD simulations were analysed to extract heatmaps of receptor-peptide interaction patterns and to determine the free energy of binding using the molecular mechanics Poisson-Boltzmann surface area (MM-PBSA) approach. The extracted free energies from MM-PBSA calculations are in qualitative agreement with experimentally determined potencies. Furthermore, the interaction patterns seen in the receptor-GLP-1 complex simulations resemble previously reported binding interactions validating the simulations. Analysing the receptor-GLP-1 analogue complex simulations, we found that the major differences between the systems stem from FA interactions and positioning of acylation in the peptide. Hydrophobic interactions between the FA chain and a hydrophobic patch on the extracellular domain contribute significantly to the binding affinity. Acylation on Lys26 resulted in noticeably more interactions between the FA chain and the extracellular domain hydrophobic patch than found for acylation on Lys34 and Lys38, respectively. The presence of a charged linker between the peptide and FA chain can potentially stabilise the complex by forming hydrogen bonds to arginine residues in the linker region between the extracellular domain and the transmembrane domain. A molecular understanding of the fatty acid structure and its effect on binding provides important insights into designing acylated agonists for GLP-1R.Communicated by Ramaswamy H. Sarma.

Implications of ligand-receptor binding kinetics on GLP-1R signalling

G protein-coupled receptors (GPCRs) are the largest class of membrane proteins and in recent years there has been a growing appreciation of the importance in understanding temporal aspects of GPCR behaviour, including the kinetics of ligand binding and downstream receptor mediated signalling. Class B1 GPCRs are activated by peptide agonists and are validated therapeutic targets for numerous diseases. However, the kinetics of ligand binding and how this is linked to downstream activation of signalling cascades is not routinely assessed in development of peptide agonists for this receptor class. The glucagon-like peptide-1 receptor (GLP-1R) is a prototypical class B1 GPCR and a validated target for treatment of global health burdens, including type 2 diabetes and obesity. In this study we examined the kinetics of different steps in GLP-1R activation and subsequent cAMP production mediated by a series of GLP-1R peptide agonists, including the ligand-receptor interaction, ligand-receptor-mediated G protein engagement and conformational change and cAMP production. Our results revealed GLP-1R peptide agonist dissociation kinetics (K_off), but not association kinetics (K_on), were positively correlated with the onset of receptor-G protein coupling/conformational change, onset of cAMP production and duration of cAMP signalling. Thus, this study advances the understanding of molecular events that couple GLP-1R ligand binding to intracellular signaling, with the findings likely to have implications for mechanistic understanding of agonist action at other related class B1 GPCRs.